SIESTA is both a method and its computer program implementation, to
perform efficient electronic structure calculations and ab initio
molecular dynamics simulations of molecules and solids. SIESTA's
efficiency stems from the use of strictly localized basis sets and
from the implementation of linear-scaling algorithms which can be
applied to suitable systems. A very important feature of the code is
that its accuracy and cost can be tuned in a wide range, from quick
exploratory calculations to highly accurate simulations matching the
quality of other approaches, such as plane-wave and all-electron
methods.

The possibility of treating large systems with some first-principles
electronic-structure methods has opened up new opportunities in many
disciplines. The SIESTA program is distributed freely to academics and has
become quite popular, being increasingly used by researchers in
geosciences, biology, and engineering (apart from those in its natural
habitat of materials physics and chemistry). Currently there are
several thousand users all over the world, and the paper
describing the method
(J. Phys. Cond. Matt. 14, 2745 (2002)) has received
more than 6000 citations so far.